Soil moisture measurement

Question

Background

To kick-start my longstanding ambition of getting back into electronics, I decided to try and make a device that measures the soil moisture of potted plants to determine whether they need to be watered. The aim for the first incarnation is to have a led that shows the plant's current water level (add water is red!), but of course I have wild plans for self-watering plants down the road.

My first stab was based on the Garduino instructable, specifically this circuit here:

It is a simple voltage divider using a known resistor and two copper wire electrodes stuck in the soil as measuring device. The voltage measured at A0 using the ADC on my Teensy++ is proportional to the ratio of the resistances, and I can work back to the actual soil resistance.

However, in practice the soil resistance turned out to be subject to a very large drift. Almost immediately after turning on the device, the measured soil resistance started dropping. (This also happens if you stick a multimeter in the plant.) After a few days of measurement, I could find no usable connection between the measurements and the moisture level. Removing the probe and inspecting it revealed considerable tarnish on the anode. To me, this indicates that some sort of chemical reaction is taking place (electrolysis?). A multimeter measures a voltage differential of a few tens of millivolts: I have turned my plant into a battery!

To avoid this problem I moved to a probe constructed out of stainless steel screws. As they are considerably less reactive than copper, the oxidation should be less of a problem. Also, to avoid "charging" the soil, i have put the voltage divider between two digital output pins of the Teensy. The measurement happens as follows:

• Both pins are kept low when no measurement is taking place.
• I drive one pin high. I wait one millisecond and I measure the voltage at the center of the divider.
• I invert the pin voltages (high<->low), wait one millisecond and measure again. I should now measure the complement of the first measurement.
• Both pins are brought low again and the device sleeps until the next measurement.

This does reduce the oxidation problem, but the measured soil resistance still seems to start dropping as soon as I start measuring.

Question

1. Can anyone recommend a robust method to measure soil moisture?
2. Can anyone explain what is really going on here?

Answer 1

Brainstorming:

I've been reading about capacitive sensing recently. Capacitive-touch sensors have the great advantage that they can be completely hermetically sealed behind a thin layer of plastic, so there is no exposed metal to corrode. ("Electronics and Robotics: Sensing touch through a thick surface?")

Because you can't get DC current to flow through the plastic, such sensors must use AC, as Leon Heller suggested.

Alas, most of the discussion I've seen has been about how to respond only to intelligent finger-presses; what little discussion there is about water an moisture is about how to reduce the sensitivity of capacitive sensors to water splashes. ("Sparkfun forums: Capacitive sensors and waterproofing"). So clearly they are sensitive to water, so perhaps you could use this "flaw" and turn it into a feature.

Answer 2

You should be using AC, to avoid polarisation of the electrodes, with a Wenner bridge arrangement.

Answer 3

The only robust method for measuring soil moisture that I am aware of is using these probes.

Answer 4

Here's how botanicalls does it.

Answer 5

I've constructed a soil humidity sensor using two galvanized nails set in a cylinder of Plaster of Paris. I didn't test it over a long enough period of time to draw any conclusions as you have, but you might experiment with that design.

The plaster neutralizes any acid in the moisture, thus isolating humidity content as a variable (but also works as a kind of low-pass filter!) + the galvanization on the nails helps to prevent them from oxidizing.

Please post if you find a good solution.

Answer 6

You might try using the Irrometer Watermark sensors. I'm using them now to measure soil moisture. If you're interested, I can try and dig up the schematic.

Answer 7

I'd recommend the Watermark sensors. They're durable and quite easy to read. You need two digital outputs plus one input per sensor.

Connect each meter to your outputs, in series with a 10k resistor. When you want to measure, set one output High and the other Low. Measure the voltage across the sensor. http://www.irrometer.com/pdf/supportmaterial/sensors/voltage-WM-chart.pdf has a chart for rough calibration; you'll need to establish a "completely wet" baseline. Because you're supposed to thoroughly wet the soil when you bury the sensor, you'd do this anyway in order to make sure the thing is working correctly.

You use two outputs because you should switch polarity in order to avoid corrosion.